In prior art inertial navigational systems employing a star tracker, the requirements of determining an accurate, arbitrarily chosen direction is accomplished by (1) either the line of sight is gimballed with respect to the stable element of an inertial system, or (2) the tracker is rigidly mounted to the stable element. A star is chosen which is located as close as possible to the desired direction and the stable element is initially aligned sufficiently close to the star direction to cause the star to be within the star tracker field of view.
The first approach is capable of sensing to an arbitrarily chosen direction as required by sighting successively on two stars and determining the orientation of the stable member in these axes. It has the disadvantage of requiring the precise measurement of large line of sight displacements in two axes. Where high accuracies are required, the error in the tracker gimbal bearings and angle measurement preclude such an approach. The second approach, since the tracker is mounted to the stable member, is highly accurate providing a suitable star is located in the desired direction. Where a star is not so conveniently positioned, significant errors are introduced, and in general, overall accuracy is limited.
The present invention is designed to obtain the arbitrary direction capability of the gimballed type of tracker combined with the optimum star position accuracy of the fixed stable member. This is accomplished along a preferred axis to an accuracy previously attainable only along an axis which points directly at a star, and to a modestly degraded accuracy in other directions. This is to be compared with previously known means which require an independent two-axis gimballing of the star tracker with respect to the inertially stable member and are subject to the errors in the angle measuring devices, or which require precessing of the platform through relatively large angles and are consequently subject to large gyro torquing errors.